Dynamic monitoring of physiological functions of patients at the bedside is highly desirable in order to minimize the risk of acute renal failure brought about by various clinical, physiological, and pathological conditions (Rabito et al., Renal function in patients at risk with contrast material-induced acute renal failure: Noninvasive real-time monitoring, Radiology 1993, 186, 851; Tilney and Lazarus, Acute renal failure in surgical patients: Causes, clinical patterns, and care, Surgical Clinics of North America, 1983, 63, 357; VanZee et al., Renal injury associated with intravenous pyelography in non-diabetic and diabetic patients, Annals of Internal Medicine, 1978, 89, 51-54; Lundqvist et al., Iohexol clearance for renal function measurement in gynecologic cancer patients, Acta Radiologica, 1996, 37, 582; Guesry et al., Measurement of glomerular filtration rate by fluorescent excitation of non-radioactive meglumine iothalamate, Clinical Nephrology, 1975, 3, 134). This monitoring is particularly important in the case of critically ill or injured patients because a large percentage of these patients face the risk of multiple organ failure (MOF), resulting in death (Baker et al., Epidemiology of Trauma Deaths, American Journal of Surgery, 1980, 144; Regel et al., Treatment Results of Patients with Multiple Trauma: An Analysis of 3406 Cases Treated Between 1972 and 1991 at a German Level I Trauma Center, Journal of Trauma, 1995, 38, 70). MOF is a sequential failure of lung, liver, and kidneys, and is incited by one or more severe causes such as acute lung injury (ALI), adult respiratory distress syndrome (ARDS), hypermetabolism, hypotension, persistent inflammatory focus, or sepsis syndrome. The common histological features of hypotension and shock leading to MOF include tissue necrosis, vascular congestion, interstitial and cellular edema, hemorrhage, and microthrombi. These changes affect the lung, liver, kidneys, intestine, adrenal glands, brain, and pancreas, in descending order of frequency (Coalson, Pathology of Sepsis, Septic Shock, and Multiple Organ Failure, New Horizons: Multiple Organ Failure Syndrome, Bihari and Cerra (Eds.), Society of Critical Care Medicine, Fullerton Calif., 1986, pp. 27-59). The transition from early stages of trauma to clinical MOF is marked by the extent of liver and renal failure and a change in mortality risk from about 30% to about 50% (Cerra, Multiple Organ Failure Syndrome. New Horizons: Multiple Organ Failure, Bihari and Cerra (Eds). Society of Critical Care Medicine, Fullerton Calif., 1989, pp. 1-24).
Serum creatinine measured at frequent intervals by clinical laboratories is currently the most common way of assessing renal function and following the dynamic changes in renal function which occur in critically ill patients (Doolan et al., A clinical appraisal of the plasma concentration and endogenous clearance of creatinine, American Journal of Medicine, 1962, 32, 65-79; J. B. Henry (Ed). Clinical Diagnosis and Management by Laboratory Methods, 17th Edition, W. B. Saunders, Philadelphia Pa., 1984); Speicher, The right test: A physician's guide to laboratory medicine, W. B. Saunders, Philadelphia Pa., 1989). These values are frequently misleading, since age, state of hydration, renal perfusion, muscle mass, dietary intake, and many other clinical and anthropometric variables affect the value. In addition, a single value returned several hours after sampling is difficult to correlate with other important physiologic events such as blood pressure, cardiac output, state of hydration and other specific clinical events (e.g., hemorrhage, bacteremia, ventilator settings and others). An approximation of glomerular filtration rate can be made via a 24-hour urine collection, but this requires 24 hours to collect the sample, several more hours to analyze the sample, and a meticulous bedside collection technique. New or repeat data are equally cumbersome to obtain. Occasionally, changes in serum creatinine must be further adjusted based on the values for urinary electrolytes, osmolality, and derived calculations such as the “renal failure index” or the “fractional excretion of sodium.” These require additional samples of serum collected contemporaneously with urine samples and, after a delay, precise calculations. Frequently, dosing of medication is adjusted for renal function and thus can be equally as inaccurate, equally delayed, and as difficult to reassess as the values upon which they are based. Finally, clinical decisions in the critically ill population are often as important in their timing as they are in their accuracy.
Exogenous markers such as inulin, iohexol, 51Cr-EDTA, Gd-DTPA, or 99mTc-DTPA have been reported to measure the glomerular filtration rate (GFR) (Choyke et al., Hydrated clearance of gadolinium-DTPA as a measurement of glomerular filtration rate, Kidney International, 1992, 41, 1595; Tweedle et al., A noninvasive method for monitoring renal status at bedside, Invest. Radiol., 1997, 32, 802; Lewis et al., Comparative evaluation of urograhic contrast media, inulin, and 99mTc-DTPA clearance methods for determination of glomerular filtration rate in clinical transplantation, Transplantation, 1989, 48, 790). Other markers such as 123I and 125I labeled o-iodohippurate or 99mTc-MAG3 are used to assess tubular secretion process (Tauxe, Tubular Function, in Nuclear Medicine in Clinical Urology and Nephrology, Tauxe and Dubovsky, Editors, p. 77, Appleton Century Crofts, East Norwalk, 1985; Muller-Suur, and Muller-Suur, Glomerular filtration and tubular secretion of MAG3 in rat kidney, Journal of Nuclear Medicine, 1989, 30, 1986). However, these markers have several undesirable properties such as the use of radioactivity or ex-vivo handling of blood and urine samples. Thus, in order to assess the status and to follow the progress of renal disease, there is a considerable interest in developing a simple, safe, accurate, and continuous method for determining renal function, preferably by non-radioactive procedures. Other organs and physiological functions that would benefit from real-time monitoring include the heart, the liver, and blood perfusion, especially in organ transplant patients.
Hydrophilic, anionic substances are generally recognized to be excreted by the kidneys (Roch-Ramel et al., Renal excretion and tubular transport of organic anions and cations, Handbook of Physiology, Section 8, Neurological Physiology, Vol. II, Windhager, Ed., p. 2189, Oxford University Press, New York, 1992; Nosco and Beaty-Nosco, Chemistry of technetium radiopharmaceuticals 1: Chemistry behind the development of technetium-99m compounds to determine kidney function, Coordination Chemistry Reviews, 1999, 184, 91). It is further recognized that drugs bearing sulfonate residues exhibit improved clearance through the kidneys (Baldas and Bonnyman, Preparation, HPLC studies and biological behavior of techentium-99m and 99mTcN0-radiopharmaceuticals based on quinoline type ligands, Nucl. Med. Biol., 1999, 19, 491; Hansen et al., Synthesis of the sulfonate and phosphonate derivatives of mercaptoacetyltriglycine. X-ray crystal structure of Na2[ReO(mercaptoacetylglycylglycylaminomethane-sulfonate)].3H20, Met.-Based Drugs, 1994, 1, 31).
Assessment of renal function by continuously monitoring the blood clearance of exogenous optical markers, viz., fluorescein bioconjugates derived from anionic polypeptides, has been developed by us and by others (Dorshow et al., Noninvasive fluorescence detection of hepatic and renal function, J. Biomedical Optics, 1998, 3, 340; Sohtell et al., FITC-Inulin as a Kidney Tubule Marker in the Rat, Acta. Physiol. Scand., 1983, 119, 313, each of which is expressly incorporated herein by reference). The main drawback of high molecular weight polypeptides is that they are immunogenic. In addition, large polymers with narrow molecular weight distribution are difficult to prepare, especially in large quantities. Thus, there is a need in the art to develop low molecular weight compounds that absorb and/or emit light that can be used for assessing renal, hepatic, cardiac and other organ functions.